1. Field of the Invention
The present invention relates to an optical multi/demultiplexing circuit used in an optical communication field, and particularly to an optical multi/demultiplexing circuit used for wavelength division multiplexing.
2. Description of the Related Art
With the recent development of dense wavelength division multiplexing (DWDM) systems, optical devices with a variety of functions which are essential for the WDM systems have been developed such as wavelength multi/demultiplexers, optical filters and optical switches.
As examples of the optical devices, the following devices are reported: an arrayed waveguide grating and lattice-form filter (see, for example, M. Oguma et al. “Passband-width broadening design for WDM filter with lattice-form interleave filter and arrayed-waveguide gratings”, IEEE Photonics Technology Letters 2002, Vol. 14, pp. 328–330); an asymmetric Mach-Zehnder interferometer (see, for example, De Merlier et al., “All-optical 2R regeneration based on integrated asymmetric Mach-Zehnder interferometer incorporating MMI-SOA”, Electronics Letters 2002, Vol. 38, pp. 238–239); a symmetric Mach-Zehnder interferometer (see, for example, Y. Hashizume et al., “Integrated polarization beam splitter using waveguide birefringence dependence on waveguide core width”, Electronics Letters 2001, Vol. 37, pp. 1517–1518); a cascaded Mach-Zehnder interferometer (see, for example, K. Suzuki et al., “PLC-based dynamic gain equalizer consisting of integrated Mach-Zehnder interferometers with C- and L-band equalizing range”, Electronics Letters 2002, Vol. 38, pp. 1030–1031); and a transversal-form filter (see, for example, T. Mizuno et al., “Dispersionless interleave filter based on transversal form optical filter” Electronics Letters 2002, Vol. 38, pp. 1121–1122).
Recently, the demand for optical devices for CWDM (Coarse Wavelength Division Multiplexing) systems used particularly in metro-network have increased remarkably (see, for example, R. R. Patel et al., “Multi-mode fiber coarse WDM grating router using broadband add/drop filters for wavelength re-use” LEOS' 99 12th Annual Meeting Vol. 2, pp. 826–827).
The wavelength grid of such a CWDM system has a uniform wavelength period of 20 nm. Accordingly, the optical devices for the CWDM system must be designed to have the passband with uniform wavelength period.
However, since the transmittance spectra of the conventional optical multi/demultiplexing circuits such as Mach-Zehnder interferometers have a uniform frequency period, its wavelength characteristics do not become periodic on a uniform wavelength axis. Consequently, they are not applicable to the CWDM systems because of their variations in an insertion loss, passband width and extinction ratio depending on the wavelength grid.
FIG. 1 shows a conventional Mach-Zehnder interferometer as a concrete example. The Mach-Zehnder interferometer consists of two optical couplers 905 and 906, an optical delay line section 907 between the two optical couplers, and two input/output optical waveguides connected to the optical couplers 905 and 906 (see, for example, K. Okamoto, “Fundamentals of optical waveguides” Academic Press 2000, pp. 159–161). As the optical couplers 905 and 906, directional couplers are used whose power coupling ratio is set at 50%.
The Mach-Zehnder interferometer is a multi/demultiplexing circuit with uniform frequency period as will be described below. The two optical output powers of the Mach-Zehnder interferometer are given by the following expressions.|A|2═|A0|2sin2(ξ/2)  (1)|B|2═|A0|2cos2(ξ/2)  (2)where A0 is the intensity of light input to one of the input ports, and ξ is the phase given by the optical delay line.
Using the relationship of f=c/λ, ξ is given by the following expression.
                    φ        =                                                            2                ⁢                π                            λ                        ⁢            n            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            L                    =                                                    2                ⁢                π                ⁢                                                                  ⁢                n                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                L                            c                        ⁢                          f              m                                                          (        3        )            where n is a refractive index, ΔL is a path length difference, f is a frequency, c is the speed of light, λ is the wavelength, and m is an integer.
The frequency period are given by the following expression using the foregoing expression (3) considering that the squares of cosine and sine functions have a period π.
                              Δ          ⁢                                          ⁢          f                =                                            f              m                        -                          f                              m                -                1                                              =                                                    c                ⁢                                                                  ⁢                π                                            2                ⁢                π                ⁢                                                                  ⁢                n                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                L                                      =                          const              .                                                          (        4        )            Thus, the frequency period become constant, which means that the Mach-Zehnder interferometer constitutes a multi/demultiplexing circuit with uniform frequency period.
FIG. 2 illustrates transmission characteristics when the central wavelength of the Mach-Zehnder interferometer is set at 1470 nm, and the optical path length difference through the optical delay line section is set at 55.9 μm that gives the frequency period of 20 nm as the demultiplexing period between the through port and cross port near the central wavelength. In FIG. 2, the horizontal axis represents wavelength, on which the wavelength grid is arranged at uniform period; the solid lines represents the transmission characteristics of the optical signal output from the cross port; and the broken lines represents the transmission characteristics of the optical signal output from the through port.
FIG. 3 illustrates transmission characteristics when the horizontal axis represents the optical frequency for comparison purposes. As illustrated in FIG. 3, the Mach-Zehnder interferometer has transmission characteristics with uniform frequency period rather than with uniform wavelength period. Although the demultiplexing periods of the through port and cross port are 20 nm near 1470 nm, and hence agree with the wavelength grid, the wavelength periods increase as they move from 1470 nm toward a longer wavelength side, thereby departing from the wavelength grid clearly. Therefore these optical devices are inappropriate for use in the CWDM system because their insertion loss, passband width and extinction ratio vary greatly depending on the wavelength grid.
In other words, since the interferometers with the conventional configurations such as the Mach-Zehnder interferometer do not have a uniform wavelength period, they present a problem in that their passband deviates from the grid on the wavelength axis as the passband departs from the central wavelength, and that the insertion losses vary remarkably depending on the wavelength.
As another example, a conventional optical multi/demultiplexing circuit has a uniform period in the optical frequency domain, and its transmission characteristics are characterized by the optical frequency period and the central optical frequency. The conventional optical multi/demultiplexing circuit, however, has only one design parameter that can be set freely in connection with the optical frequency period and central optical frequency: that is, the optical path length difference of the optical delay line. Therefore, the optical frequency period and the center optical frequency could not be set simultaneously.
Therefore the optical multi/demultiplexing circuit, which has the uniform frequency period in principle, cannot set the optical frequency period and the central optical frequency at the same time, thereby offering a problem of deteriorating the transmission characteristics even when it is used in the optical frequency domain.